Thursday, January 28, 2010

BIRD FOOD (AVIAN-EATING BEARS)

Puffins on Ninagiak Island - brown bears will swim to the island from the Katmai coast in order to take exploit ground nesting sea birds. Photo by Scott W. Michael.

Birds are not staple fare in most grizzly bear diets. But being the opportunists that they are, these bruins will sometimes feed on the eggs and chicks of ground nesting birds. The first report of bird-eating I could find in the literature comes from a paper on the brown bears of Kamchatka (Bergman 1936). This naturalist reports that Russian U. arctos enter marshy areas before the salmon run to raid the nests of water fowl (i.e., “wild ducks”).

In a more recent reports, grizzlies in the Canadian Arctic Region (Tuktoyaktuk Peninsula) were observed eating the eggs of snow geese (Chen caerulescens) and adult ptarmigan (Lagopus spp.) (the latter is a rare event), while coastal brown bears, along the Katmai coast, have been observed capturing seagulls that were attempting to share a fish meal. In the “lower 48,” Gunther and Renkin (1989) observed Yellowstone grizzlies attempting to capture ducks (Anatidae), Canadian geese (Branta canadensis) and sandhill cranes (Grus canadensis). This occurred on rare occasions and the bears were never seen to successfully capture avian prey. Even so, there is no doubt that they do occasionally capture and eat these birds.

Campbell (1991) found that brown bears invaded the nests of the dusky Canada goose (Branta canadensis occidentalis) on the Copper River Delta, Alaska. About 50 % of the nests in this area succumb to predator activity and subadult and female brown bears with young were implicated in over half of all nests destroyed (males and solitary females rarely enter the nesting area). The bears would enter the nesting areas as soon as the geese began nesting until mid to late summer, at which time they moved to salmon streams.

Brown bears greatly impact the distribution of certain seabirds on the islands along the Katmai coast. Here, U. arctos has been observed swimming relatively long distances to gain access to ground nesting birds. For example, in Hallo Bay, Katmai National Park, brown bears will swim the 3.2 km (2 mi.) to Ninagiak Island to feed on the eggs of glaucous-winged gulls (Larus glaucescens) and puffins (Fratercula corniculata and F. cirrhata). Bailey and Faust (1984) reported the following at another island in the area:

“A sizeable tufted puffin colony…was destroyed by a brown bear. When we visited this island.. only about 100 puffins were milling about the grassy, burrow-ridden headlands, and a bear was systematically excavating burrows around the island’s perimeter. Entire slopes were dug up to the depths of nest chambers, and eggs shells and feathers were common.”

Puffins often nest in colonies. They dig burrows that are usually around 1 to 1.2 m (3 or 4 ft.) deep (there are reports of tufted puffins digging burrows as deep as 2.75 m [9 ft.]). In some areas, like Ninagiak Island, the burrows are dug on hillsides among scattered rocks and boulders. Brown bears use their long claws and massive shoulder muscles to displace rocks and earth in order to penetrate the nesting chamber. Both Alaskan species of puffin lay a single egg that hatches in 42 to 47 days. The chicks fledge for another 45 to 55 days, remaining in the burrow this entire time. While one parent takes care of the egg or chick, the other goes out to fish for food. Eggs and young puffins are available as bear food from at least June to August or September. On islands frequented by bears, puffins are rare.

Gull populations can also be impacted by the presence of coastal brown bears. Bailey and Faust (1984) report that gull nests were often found destroyed by bears and that bruins limit where these bird’s reproduce. They conclude that:

“The ubiquitous bears probably are largely responsible for the fact that fewer seabirds nest between Kamishak and Amber Bays than along any similar length of coastline on the Alaska Peninsula.”

The glaucous-winged gull, which is targeted by brown bears on Ninagiak Island, usually scrapes its nest in the ground and fills it with bits of grass, weed, moss, roots, twigs, turf, seaweed, etc. To give you some idea how profitable gulls can be as a food source for bears, consider this. Glaucous-winged gulls form nesting colonies which can number from 10 to as many as 10,000 pairs. Sometimes other gulls also form part of these large nesting aggregations. From 1 to 3 eggs are laid. They hatch in mid- to late June and are raised within the nesting territory.

During their study, Bailey and Faust (1984) observed 40 brown bears on or swimming between islands. They reported that many of these were sows and their cubs. It is advantageous for a mother bear to take her offspring to one of these offshore islands, not only because of the ready supply of “bird-food,” but also the island is likely to provide a safer refuge away from marauding male conspecifics. It is likely that cubs that were taken to an island by their mother, will return with their out young in time.

References:

Bailey, E. P. and N. H. Faust. 1984. Distribution and abundance of marine birds breeding between Amber and Kamishak Bays, Alaska, with notes on interactions with bears. Western Birds 15:161-174.

About Me

I am into animals (especially those that live in marine environs, grizzly bears and dogs) and have been blessed to be able to make a living sharing my love with others. I have written 11 books on sharks, rays, coral reef fishes and marine invertebrates and have had thousands of photos published worldwide in books, magazines and on the internet. I am currently writing for a number of aquarium publications.